Graduate Thesis Or Dissertation
 

Pathways and enzymes involved in glucose catabolism by lactic streptococci

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  • The enzymes and pathways involved in the catabolism of glucose by several strains of Streptococcus diacetilactis, Streptococcus cremoris, and Streptococcus lactis, commonly called the lactic streptococci, were studied. The presence of aldolase, triosephosphate isomerase and alcohol dehydrogenase in these organisms provided evidence for the operation of the Embden-Meyerhof-Parnas (EMP) pathway for the catabolism of glucose, resulting in the formation of ethanol in addition to large amounts of lactic acid. Results of manometric experiments showed that the lactic streptococci had the capability to carry out the oxidative decarboxylation of pyruvate and other alpha-keto acids (alpha-ketobutyric, alpha-ketovaleric, alpha-ketoisovaleric, alpha-ketocaproic and alpha-ketoisocaproic acid) possibly by NAD-lipoate linked pyruvate dehydrogenase catalyzed reactions. The existance of operative hexosemonophosphate (HMP) pathway in the lactic streptococci was demonstrated as a result of the discovery of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase in these organisms. This pathway may provide the lactic streptococci with pentoses and dihydronicotinamide adenine dinucleotide phosphate (NADPH), which may be used in biosynthetic reactions. Acetate kinase was found to be present in the lactic streptococci. The presence of this enzyme provided evidence for the occurrence of phosphoroclastic reaction involving phosphotransacetylase and acetate kinase in these organisms. This reaction sequence converts acetyl coenzyme A to acetate via the intermediate formation of acetyl phosphate resulting in the ultimate generation of one molecule of adenosine triphosphate from one molecule of acetyl coenzyme A. The presence of acetate kinase also provided evidence for the possible operation of the phosphoketolase catalyzed phosphorolytic cleavage of xylulose-5-phosphate, an intermediate of the HMP pathway, to form triose phosphate and acetyl phosphate Acetyl phosphate may then form acetate and ATP through the acetate kinase catalyzed reaction. The radiorespirometric pattern for the catabolism of specifically labeled glucose by the various species and strains of lactic streptococci was found to be essentially the same and showed the actual operation of both EMP and HMP pathways for the catabolism of glucose by these organisms. The EMP pathway was however found to be the main route of the catabolism of glucose by these organisms. The radiorespirometric experiments also demonstrated the absence of operative tricarboxylic acid cycle in these organisms. This investigation of the enzymes and pathways involved in the catabolism of glucose by lactic streptococci helped in obtaining a proper understanding of the processes by which glucose is utilized for biosynthesis and energy production in these organisms. This investigation also provided evidence against the classification of S. diacetilactis, S. cremoris, and S. lactis in a strictly homofer, mentative group. The classification of these organisms in the microaerophilic group, which includes organisms that grow best in the presence of very limited amount of oxygen, has also become questionable because of the presence of an active oxidative decarboxylation mechanism of pyruvate decarboxylation in these organisms. Also the presence of excess oxygen does not inhibit in any way the growth of S. diacetilactis M-8-224(a).
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